The present invention relates generally to medical devices and more specifically relates to implantable electronic devices for muscle stimulation.
The earliest implantable pacing systems operated asynchronously to normal physiologic functions. U.S. Pat. No. 3,057,356, issued to Greatbatch, teaches such a pacemaker which includes a fixed rate oscillator which determines an escape interval. At the expiration of each escape interval, a pacing pulse is generated. Subsequent designs, such as in U.S. Pat. No. 3,478,746 issued to Greatbatch, incorporated a sense amplifier coupled to the pacing electrode, which sensed the electrical activity indicative of a contraction of the heart and reset the oscillator to restart timing the escape interval. These pacemakers, called demand pacemakers, only paced if no natural contractions were sensed within the escape interval.
As an alternative to regulating pacing rate by sensing the contractions of the heart, some pacemakers have regulated rate in response to measurement of some other, physiological parameter. There have been pacemakers proposed which vary rate in accordance to instantaneous blood pressure within the right atrium, as in U.S. Pat. No. 3,358,690, in response to respiration as in U.S. Pat. No. 3,593,718, in response to physical activity as in U.S. Pat. No. 4,140,132 or in response to neurological activity as in U.S. Pat. No. 4,210,219. The most promising techniques appear to involve varying of pacing rate in response to sensing of chemical parameters of the blood. For example, U.S. Pat. Nos. 4,009,721 and 4,252,124 teach pacemakers in which an implantable pH sensor determines the rate of the pacing oscillator. U.S. Pat. No. 4,202,339 issued to Wirtzfeld and U.S. Pat. No. 4,399,820 issued to Wirtzfeld et al, both teach a pacing system in which the rate of an asynchronous pacing oscillator is controlled by the oxygen level of the intracardiac venous blood. U.S. Pat. No. 4,467,807 issued to Bornzin combines the techniques of varying the rate of the pacemaker in response to sensed oxygen with the demand function, so that an interaction of both of these factors determines the delivery of pacing impulses by the pacemaker.